Many laser printers employ a continuously moving printing drum because a large amount of data is output to the printing device in a rapid, continuous process. The printer cannot stop the drum mechanism precisely enough to wait until more data arrives, without creating a visible gap or misalignment of the dots on the printed page.
Rotating polygon scanning mirrors are used in one method of laser printing to provide a raster scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep, which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line. Since the movement of the rotating drum is substantially constant and does not stop as each sweep or scan line traverses from one side to the other, it will be appreciated that the sweep or scan can not be exactly at a right angle with the rotating drum or the lines of print would be at a slight angle on the finished printed page. Instead, the rotating polygon mirror is mounted at a slight angle so that the scanning light beam sweeps or scans at a slight angle with respect to the movement of the rotating drum. However, since the speed of the rotating polygon mirror and the rotating photosensitive medium or drum are synchronized, and since the slight angular movement of the sweep or scan advances the location of the beam image the same distance and in the same direction as the movement of the rotating photosensitive medium or drum, the resulting lines of print are parallel and substantially at right angles or orthogonal to the printed page.
This arrangement will maintain parallel printed line images so long as the relative geometries of the rotating polygon mirror and photosensitive drum remain constant from scan to scan. Typical polygon scanning mirror laser printers do not include any means or devices for dynamic alignment of the rotating mirror and the photosensitive drum. It is further appreciated by those skilled in the laser printing art, that the rotating mirror is a very precise part or component of the laser printer, which must spin at terrific speeds and without undue wear of the bearings if its print quality is to be maintained.
A prior art single flat mirror configuration that prints in one direction only may be less complex and lighter than a polygon scanning mirror, however the single flat mirror, single direction configuration is inefficient as there is no printing during the mirrors return scan. This method may require about two times the on and off switching speed of the laser compared to a polygon based laser printer because of the low duty cycle of the scan engine. Further a prior art single flat mirror operated in both directions may cause a small decrease in print quality due to the zigzag effect. The zigzag effect pertains to a print difference between the edges of a page and the center of a page.
Another prior art method is a two-axis analog mirror MEMS device, which may or may not be fabricated out of a single piece of material (such as silicon, for example). A two-axis analog mirror MEMS device is smaller and lighter than a polygon mirror configuration. A two-axis mirror MEMS device may be used to minimize or eliminate the zigzag effect of a prior art single axis MEMS device. The layout of a two-axis analog mirror MEMS device may consist of a mirror supported on a gimbal frame by two silicon torsional hinges. The gimbal frame is attached to a support frame by another set of torsional hinges. One example, of a dual axis torsional hinged mirror is disclosed in U.S. Pat. No. 6,295,154 entitled “Optical Switching Apparatus” and was assigned to the same assignee on the present invention. While the two-axis mirror MEMS device may be effective in minimizing or eliminating the zigzag effect, the two-axis mirror may be expensive to manufacture.